Thermal relay device



Sept. 1, 1959 F. A. RECORD THERMAL RELAY DEVICE Filed March 17, 1958INVENTOR. FRANK A. RECORD United Sttes Patent 6 THERMAL RELAY DEVICEFrank A. Record, Danvers, Mass., assignor to Bomac Laboratories Inc.,Beverly, Mass., a corporation of Massachusetts Application March 17,1958, Serial No. 722,028

2 Claims. (Cl. 200-117) The present invention relates to thermal relaysand has particular reference to a miniaturized reliable structure forapplications involving a wide range of environmental conditions such astemperature, vibration and shock.

Devices of the character to be described may be employed in cooperationwith numerous electrical circuits to activate any desired mechanism on aone-shot basis Where performance capable of high reliability isrequired. A further requirement for the intended usage is that subjectdevices be miniaturized without in any way sacrificing any of thereliability features.

It is an object of the present invention to provide a miniaturizedthermal relay of simplified and ruggedized design.

A further object is the provision of a thermal relay having a highdegree of reliability under adverse environmental conditions.

The invention features a plurality of sets of conductive contactingmembers which in the final state of operation mate with one another toprovide a conductive path to either trigger or close related electricalcircuitry. Initially, one set of contacting members are supported undertension by a fusible element which is rapidly burned-out by passage ofan electrical current through suitable leads. The overall relaystructure is hermetically sealed in a dielectric enclosure with externalleads extending therethrough for connection to desired circuitry.Suitable inert atmospheres such as dry nitrogen, argon and helium may beintroduced into the enclosure or it may be evacuated. The advantagesinclude non-critical adjustments of contact members, ease and speed ofassembly by use of stamped components and self-alignment of thestructure. The provision of parallel conductive paths assures the highreliability of operation. Further, selection of appropriate hightemperature metals and method of assembly has resulted in reliability ofoperation under severe conditions of temperature, vibration and shock.

Further objects, features and advantages will be apparent afterconsideration of a detailed description of an illustrative embodimentand the accompanying drawings, in which:

Fig. 1 is an enlarged perspective view with a portion of the envelopebroken away;

Fig. 2 is a detailed cross-sectional view;

Fig. 3 is an exploded view of the internal contact assembly structure;and

Fig. 4 is an enlarged View of a portion of the illustrative device toreveal structure for mounting of the fusible and tensioning elements.

Referring now to the drawings, in particular Figs. 1 and 2 theillustrative embodiment comprises an envelope 1 with glass stems 2 and 3sealed at the ends thereof. Each stem defines a hub portion 4 and 5 withelectrical leads extending therethrough. Leads 6 and 7 are supported bystem 2 and another pair of leads 8 and 9 are supported by the oppositelydisposed stem 3. If desired other types of stem enclosures such as thedielectric reen- 2,902,566 Patented Sept. 1, 1959 trant or butt sealtypes or metallic headers may be employed at the ends of envelope 1 inthe manner well known in the art.

An upper contact assembly 10 may be fabricated from a flat stock of ametal, such as beryllium copper or molybdenum and may also be platedWith a coating of any highly conductive metal, such as gold. Contactingassembly i0 is provided with split rim section 11 and a pair of arms 12and 13 depending downwardly and then angularly therefrom to define a gaptherebetween. Rim 11 is formed in such a manner that it will firmlyengage the hub portion 4 to which it is secured.

Another or lower contact assembly 14 fabricated from a similar metal asassembly 10 is mounted on hub portion 5 of the bottom stem 3. A rim 15to firmly grip the hub portion is provided and arms 16 and 17 extendtherefrom so as to contact one another under tension. To assure goodelectrical contact when the relay device is in the fired stage ofoperation, the fiat surfaces adjacent to the ends of arms 16 and 17 maybe provided with embossmerits 18. A third arm 19 is defined in assembly14 for use in connection to external leads in a manner to be describedhereinafter.

The operation of the illustrative thermal relay device and the method ofassembly will be better understood by referring now to Figs. 3 and 4. Itwill also be possible to follow the connection of the external leads tointernal contact members by means of dotted lines shown in Fig. 3. Lead6 is electrically connected to arm 19 and lead 7 is secured to contactassembly 10. These leads, therefore, provide means for connection to anexternal circuit. With the arms 16 and 17 of contact assembly 14 incontiguous relationship by means of the fusible element and tensioningarrangement to be described, the contacting arms will be positioned inthe space defined between upper contact arms 12 and 13. This representsthe initial state of the device and as a result no conductive path wouldbe defined between leads 6 and 7. After removal of the tensioning meansarms 16 and 17 snap apart and mate with upper arms 12 and 13 to completethe circuit.

The fusible element and tensioning means will now be described,reference being directed to Fig. 4. The leads 8 and 9 are formed attheir inner ends with right angular extensions oppositely disposed as at20. An insulator 21 of a material such as a ceramic is then secured toextensions 20 by means of a fusible wire 22 such as for example a wireof a material, commonly referred to by the trade-mark Karma, withseveral turns around each lead extension and the ends secured as bywelding. An axial passageway 25' in insulator 21 extending parallel toextensions 20 provides means for introduction of a tensioning wire 23which is passed through opening 24 in the arms 16 and 17 several timesand then secured at the ends thereof to said arms. In an illustrativeembodiment a nickel ribbon .003 thousandths thick and .010 thousandthswide was employed because of the ease in spotwelding the ends thereof tothe arms.

In the method of assembling the device, the insulator 21 with thetensioning wire extending therethrough and secured to arms 16 and 17 ispulled downward to bring the ends of said arms in contact. The fusiblewire 2-2 is then secured around the insulator and extensions 20 of leads8 and 9. Hence by the passage of an electrical current through leads 8and 9 the fusible wire will be burned out to thereby result in arms 16and 17 snapping apart to mate with arms 12 and 1'3 of the upper contactassembly. With the structure disclosed high reliability has beenattained in relays required to operate on a one-shot basis. Further,such devices have been fabricated having an overall length of only of aninch and a diameter of only A of an inch which is highly desirable,particularly in air-borne applications. Other possible fusible elementarrangements will occur to those skilled in the art. For example, aninsulator in the form of a grooved pulley could be suspended by means ofa tensioning wire between contact arms 16 and 17. A fuse wire would thenride over the pulley and be secured to the ends of the leads to definethe conductive path for passage of a burn-out current. A furthermodified insulator may be provided in the form of a hook and eyecombination with the hook portion coated with an insulating materialsuch as sauereisen glass. The burnout wire would then be positioned overthe hook WhiiC the tensioning wire would extend through the eye portion.

In addition the invention may be provided with an c:-;- haust tubulationintroduced in either of the end stems. Either evacuation or filling ofthe envelope with an inert atmosphere would then be possible. Argon,helium or dry nitrogen have been successfully employed with the latterdisplaying excellent hold-oil voltage characteristics.

What is claimed is:

1. A thermal relay device comprising an envelope hermetically sealed atthe ends thereof by stern enclosures, first and second oppositelydisposed metallic contact assemblies supported by said stem enclosures,said first contact assembly comprising a pair of dependent arms definingan open gap therebetween, said second contact assembly comprising a pairof dependent arms held in contiguous relationship by tensioning andfusible means with the ends of said fusible means connected toelectrical leads extending through one of said stem enclosures, saidfusible means being rapidly severed by passage of an electrical currenttherethrough to thereby cause said concontact assemblies of a highlyconductive resilient metal,

said first contact assembly comprising a split-rim section girdling oneof said hub sections, a pair of arms extending from said rim sectionwith the ends thereof spaced apart to define a gap therebetween, saidsecond contact assembly comprising a split-rim section girdling theoppositely disposed hub section, a pair of arms extending from said rimsection with the ends of said arms held in contiguous relationship bytensioning means and a fusible wire, electrical leads extending throughthe hub portion supporting said second contact assembly with saidfusible Wire being connected to said leads, said fusible Wire beingrapidly severed by passage of an electrical current therethrough tothereby cause said contiguous arms to spring apart and mate with thearms of said first contact assembly and electrical leads extendingthrough the opposite hub portion for connection to each of said contactassemblies to define a continuous conductive path when said contactassemblies are in mating relationship.

Miller et al. July 6, 1954 Sims et al. May 7, 1957

